The invention relates to a method and arrangement in connection with a half-controlled network bridge. More specifically, the invention relates to a method and arrangement for controlling thyristors of a half-controlled network bridge.
A network bridge is an electric converter for converting ac voltage of a network to dc voltage. A network bridge in its simplest form has six diodes, with two diodes connected in series for each network phase. These series-connected diodes are further connected in parallel with each other. Network phases are connected between the series-connected diodes, the anodes and cathodes of the parallel-connected pairs forming a positive and a negative terminal for dc voltage. This type of connection can be used for generating a 6-pulse voltage, the magnitude of which cannot be changed.
Half-controlled network bridges are commonly used in connection with frequency converters provided with intermediate DC circuits for generating the dc voltage of the intermediate circuit. The magnitude of the voltage generated by a half-controlled network bridge can be controlled by using controllable switches of the bridge. FIG. 1 illustrates an example of a half-controlled network bridge composed of three diodes and three thyristors. Each connected diode and thyristor forms a series-connected pair in which the cathodes of the diodes are connected to the anodes of the thyristors. Further, all pairs thus connected are further connected in parallel and the network voltage phases are connectable between the series-connected components. A rectified voltage Udc is formed between the cathodes of the thyristors and the anodes of the diodes.
A thyristor is known to be a component that can be switched on to a conductive state by supplying gate current to the gate, when the thyristor voltage is forward-biased. However, a conventional thyristor cannot be switched off from the gate, but the thyristor remains conductive for as long as there is current passing through it. The operation of thyristors in connection with a network rectifier of a frequency converter is important for generating the desired intermediate circuit dc voltage. Therefore, to ensure that thyristors turn on, they are not only supplied with a single gate current pulse sufficient for turn-on, but also receive continuous current feed for as long as turn-on is possible.
Prior art teaches gate control achieved by connecting the gate to a voltage, the gate current provided by which being restricted by using resistive coupling. To reduce power consumption, voltage is switched to the gate, thus producing a pulsed gate current. A typical example of a prior art implementation is to produce a pulsed gate current at a predetermined switching frequency. An example of this kind of constant switching frequency is 30 kHz, with a current amplitude of 0.5 A and pulse rate of about 55/45. Each cycle thus has 20 μs of current-carrying time and about 16 μs dead time. However, the gate current generated this way involves considerable power consumption, because the magnitude of the current is restricted by a resistor.
In the prior art method, the gate current depends on the voltage connected to the gate, on the resistances on the current path and on the voltage drops. Consequently, the magnitude of the current may vary significantly due to variations in the supply voltage and in the gate-cathode voltage of the thyristor to be controlled. In the worst case, the current is not sufficient for turning on the thyristor as desired.
Moreover, continuous switching of the gate current circuit may cause problems relating to electromagnetic disturbances due to high switching-off rates of the gate current.
US 2002/0044004 A1 proposes a solution in which a continuous gate current is delivered to turn on a thyristor. However, the solution is complicated when used for controlling thyristors in a half-controlled thyristor bridge.
U.S. Pat. No. 7,388,765 B2 discloses a method in which a continuous current is used to turn on the thyristors. This continuous current is further obtained from a voltage source that is common to all gate driver circuits. The problem with the method disclosed in U.S. Pat. No. 7,388,765 relates to the large number of components for carrying out the required operations.